Page not Found (404) and it's get tricky

Using Django Mystery Errors
1

Hello, everyone, I am using django 4.2.19, when I runserver and go to “about” page or “contact” I see an error Page not found (404) raised by blog.views.PostDetailView.
So this is my code:

# views.py
from django.views import generic
from django.conf import settings
from django.core.mail import send_mail
from django.shortcuts import reverse
 
from .models import Post
from .forms import ContactForm
 
 
class PostListView(generic.ListView):
    model = Post
    queryset = Post.objects.all()
    context_object_name = 'posts'
    template_name = 'post_list.html'
    paginate_by = 5
 
 
class PostDetailView(generic.DetailView):
    model = Post
    context_object_name = 'post'
    template_name = 'post_detail.html'
 
 
class SuccessView(generic.TemplateView):
    template_name = 'success.html'
 
 
class ContactView(generic.FormView):
    form_class = ContactForm
    template_name = 'contact.html'
 
    def get_success_url(self):
        return reverse('blog:contact')
 
    def form_valid(self, form):
        email = form.cleaned_data.get('email')
        subject = form.cleaned_data.get('subject')
        message = form.cleaned_data.get('message')
 
        full_message = f"""
            Received message below from {email}, {subject}
            ______________________
            {message}
        """
        send_mail(
            subject='Received contact form from submission',
            message=full_message,
            from_email=settings.DEFAULT_FROM_EMAIL,
            recipient_list=[settings.NOTIFY_EMAIL],
        )
        return super(ContactView, self).form_valid(form)


class AboutView(generic.TemplateView):
    template_name = 'about.html'

# urls.py
from django.urls import path
 
from .views import PostListView, PostDetailView, ContactView, SuccessView, AboutView
 
app_name = 'blog'
 
urlpatterns = [
    path('', PostListView.as_view(), name='post_list'),
    path('<slug:slug>/', PostDetailView.as_view(), name='post_detail'),
    path('contact/', ContactView.as_view(), name='contact'),
    path('success/', SuccessView.as_view(), name='success'),
    path('about/', AboutView.as_view(), name='about'),
]

# models.py
from django.db import models
from django.urls import reverse
from django.utils import timezone
from django.template.defaultfilters import slugify
 
from tinymce.models import HTMLField
 
 
class Post(models.Model):
    title = models.CharField(max_length=255)
    description = models.CharField(max_length=255)
    slug = models.SlugField(null=False, unique=True)
    body = HTMLField()
    pub_date = models.DateField(default=timezone.now)
 
    class Meta:
        ordering = ['-pub_date']
        verbose_name = 'Post'
        verbose_name_plural = 'Posts'
 
    def __str__(self):
        return self.title
 
    def get_absolute_url(self):
        return reverse('blog:post_detail', kwargs={'slug': self.slug})
 
    def save(self, *args, **kwargs):
        if not self.slug:
            self.slug = slugify(self.title)
        return super().save(*args, **kwargs)

# python3 manage.py runserver
System check identified no issues (0 silenced).
February 21, 2025 - 13:22:35
Django version 4.2.19, using settings 'config.settings'
Starting development server at http://127.0.0.1:8000/
Quit the server with CONTROL-C.
 
Not Found: /contact/
[21/Feb/2025 13:22:47] "GET /contact/ HTTP/1.1" 404 2612
Not Found: /favicon.ico
[21/Feb/2025 13:22:47] "GET /favicon.ico HTTP/1.1" 404 3330

Page not found (404)
 
No Post found matching the query
 
Request Method:     GET
Request URL:    http://127.0.0.1:8000/contact/
Raised by:  blog.views.PostDetailView
 
Using the URLconf defined in config.urls, Django tried these URL patterns, in this order:
 
    admin/
    [name='post_list']
    <slug:slug>/ [name='post_detail']
 
The current path, contact/, matched the last one.

But when I change urls.py like this

# urls.py
from django.urls import path
 
from .views import PostListView, PostDetailView, ContactView, SuccessView, AboutView
 
app_name = 'blog'
 
urlpatterns = [
    path('', PostListView.as_view(), name='post_list'),
    path('contact/', ContactView.as_view(), name='contact'),
    path('success/', SuccessView.as_view(), name='success'),
    path('about/', AboutView.as_view(), name='about'),
    path('<slug:slug>/', PostDetailView.as_view(), name='post_detail'),
]

Everything is just working… Why is that?

2

Because urls are searched in the order that they appear within the urlpatterns.

Since “contact”, “success” and “about” would all be valid values for a slug, it was matching the slug pattern first, and so trying to call the PostDetailView.

See URL dispatcher | Django documentation | Django for more details.